In recent years, a small-size loudspeaker unit has been proposed, in which the minimum resonance frequency of the loudspeaker unit is reduced to several hundreds of Hz by using a magnetic fluid. In televisions and mobile phones, use of such a loudspeaker unit can increase the low frequency band characteristics, as compared to conventional loudspeaker units. However, when the loudspeaker unit having the reduced minimum resonance frequency is used in equipment such as earphones in which the loudspeaker unit is driven in a closed space surrounded by an eardrum and an external auditory canal, the low frequency band characteristics become excessive as compared to the high frequency band characteristics, and therefore, the sound pressure frequency characteristics need to be adjusted in some way.
As a method for adjusting the sound pressure frequency characteristics of the conventional earphones, a method has been proposed, in which a space is provided at a back surface of the loudspeaker unit. As a prior art literature relating to the present disclosure, for example, Patent Literature 1 has been known, which discloses a configuration of an earphone in which a space is provided on a back surface of a loudspeaker unit in a housing in which the loudspeaker unit is installed, and the volume of the space provided at the back surface of the loudspeaker unit is adjusted to adjust the sound pressure frequency characteristics.